1. Field of the Invention
The present invention relates generally to a transistor and a semiconductor structure, and more specifically to a transistor and a semiconductor structure that has a metal oxide layer on a work function metal layer.
2. Description of the Prior Art
Poly-silicon is conventionally used as a gate electrode in semiconductor devices such as metal-oxide-semiconductors (MOS). With the trend towards scaling down semiconductor devices, conventional poly-silicon gates face the problem of lower performance due to boron penetration and unavoidable depletion effect, which increases the equivalent thickness of the gate dielectric layer, reduces the gate capacitance, and worsens the driving force of the devices. Therefore, work function metals that are suitable to be used as high-K gate dielectric layers are employed to replace the conventional poly-silicon gate as the control electrode.
A method of forming a metal gate by replacing a conventional polysilicon gate with a work function metal includes: a sacrificial gate is formed on a substrate; a spacer is formed on the substrate beside the sacrificial gate; a source/drain region is formed and automatically aligned by using the spacer; an interdielectric layer is disposed and planarized on the substrate; the sacrificial gate is removed to form a recess, and then a work function metal layer, a barrier layer and aluminum are sequentially filled into the recess to form a metal gate.
As sizes of semiconductor components are reduced, material layers such as a barrier layer having a large enough thickness to prevent aluminum from diffusing downwards will be filled into the recess after the work function metal layer is filled. As part of the volume of the recess and the opening size of the recess are occupied, there will be a difficulty in filling the recess with aluminum. Furthermore, as the sizes of the semiconductor components are reduced, the volume and the exposed surface area of aluminum will also be reduced, so that the contact resistance between the aluminum and a contact plug formed above increases. As the semiconductor components are formed precisely, the electrical demand is critical. How to improve the work function values of the semiconductor components therefore becomes an important issue.